Cellulose nanofibers are a basic skeleton material (basic element) of all plants. In plant cell walls, cellulose nanofibers are present in the form of a bundle of several cellulose microfibrils (single cellulose nanofibers) having a width of about 4 nm.
Various methods are known as a method of producing cellulose nanofibers from plant fibers, etc. Generally, cellulose nanofibers are produced by defibrating or breaking up a cellulose fiber-containing material such as pulp by milling or beating, using devices such as a refiner, a grinder (stone-type grinder), a twin-screw kneader (twin-screw extruder), or a high-pressure homogenizer.
It is known that when the assembly of the cellulose nanofibers obtained by these methods is formed into a sheet, or when the cellulose nanofibers are mixed with resin to form a resin composite, the strength of the sheet or resin composite increases as the ratio (aspect ratio) of the fiber length to the fiber diameter (width) of the cellulose nanofiber increases. For example, Japanese Examined Patent Publication No. S48-6641 and Japanese Examined Patent Publication No. S50-38720 disclose a method of forming microfibril fibers utilizing a hydrophilic property, which is a feature of pulp or a cellulose fiber to obtain a cellulose-based fiber having a high aspect ratio. In these references, microfibril fibers are obtained by highly and repeatedly milling or beating up pulp using a refiner, and additionally a homogenizer, etc.
On the other hand, when pulp is defibrated, defibration is generally performed in the presence of water. After defibration, the water drainage time to separate water and the resulting cellulose nanofibers lengthens as the aspect ratio of the cellulose nanofibers increases. Specifically, to obtain a cellulose nanofiber sheet or cellulose nanofiber resin composite having a high strength, it is desirable to defibrate cellulose nanofibers having a high aspect ratio. However, when the fiber diameter is small and the aspect ratio is large, the water drainage time lengthens, which increases costs from an industrial viewpoint.
For example, in Patent Literature 1, absorbent cotton is defibrated by a high-pressure homogenizer to obtain a microfibrillated cellulose. However, when a starting material fiber, such as pulp, is defibrated by a high-pressure homogenizer, the fiber diameter is generally reduced to increase the aspect ratio. Therefore, although the high sheet strength can be obtained the water drainage time in the production of the cellulose nanofiber sheet becomes extremely long, which is not industrially preferable.
Patent Literature 2 discloses a method of defibrating pulp using a grinder or a twin-screw extruder. When milling is performed by a grinder, the fiber diameter is generally reduced to increase the aspect ratio; therefore, the sheet strength can be increased. However, this method also requires a relatively long water drainage time, and it is therefore not industrially preferable. The defibration by a twin-screw extruder is usually performed at a rotation speed of 200 to 400 rpm. (Since the screw diameter is 15 mm, the circumferential speed is 9.4 m/min. to 18.8 m/min.) For example, in Patent Literature 2, defibration is performed for 60 minutes at 400 rpm (circumferential speed: 18.8 m/min.). However, under such conditions, a high shear rate is not applied to pulp, and breakage of fiber advances preferentially over fiber defibration; therefore, microfibrillation (nanofiber formation) is insufficient, and it is difficult to obtain a nanofiber having high sheet strength.
In Patent Literature 3, pulp subjected to preliminary defibration using a refiner is defibrated using a twin-screw extruder at a screw rotation speed of 300 rpm, (Since the screw diameter is 15 mm, the circumferential speed is 14.1 m/min.), thus performing fine fibrillation. However, as described above, under such conditions, a high shear rate is not applied to pulp, and breakage of fiber advances preferentially over fiber defibration; therefore, microfibrillation (nanofiber formation) is insufficient, and it is difficult to obtain a nanofiber having high sheet strength.